Cellulose coated carriers

ABSTRACT

An electrostatographic developer mixture comprising finelydivided toner particles electrostatically clinging to the surface of carrier beads, each of the carrier beads comprising a core having a uniform coating thereon comprising a polymeric cellulose ester wherein said coating exhibits a volume resistivity of between about 1010 and about 1014 ohm-centimeters at 23*C. The process for employing the developer mixture in developing an electrostatic latent image is also disclosed.

United States Patent 1191 Hagenbach CELLULOSE COATED CARRI ERS [75]Inventor: Robert Joseph Hagenbach,. Rochester, NY.

[63] Continuation of 'Ser. No. 18,316, Feb. 5, 1970,

abandoned.

.[52] U.S. Cl 1l7/l7.5, 96/1 SD, 117/100 M, 117/100 S, 252/621 [51] Int.Cl. 603g 9/02, 603g 13/08 [58] Field of Search ll7/17.5, 100 M, 100 S;96/1 R, 1 SD; 252/621 [56] References Cited UNITED STATES PATENTS.Bolton 117/17.5

1451 Nov, 26, 1974 2,874,063 Greig 117/17.5

2,919,247 12/1959 Allen 252/621 3,143,508 8/1964 Kaprelian 252/6213,239,465 3/1966 Rheinfrank 252/621 3,526,533 9/1970 Jacknow et a1.l17/l7.5 3,533,835 10/1970 Hagenbach et a1. 252/621 12/1970 Miller252/621 I Primary ExaminerMichael Sofocleous [57] ABSTRACT Anelectrostatographic developer mixture comprising finely-divided tonerparticles electrostatically clinging to the surface of carrier beads,each of the carrier beads comprising a core having a uniform coatingthereon comprising a polymeric cellulose ester wherein said coatingexhibits a volume resistivity of between about 10 and about 10ohm-centimeters at 23C. The process for employing the developer mixturein developing an electrostatic latent image is also disclosed.

6 Claims, No Drawings CELLULOSE COATED CARRIERS This application is acontinuation application of Ser.

The formation and development of latent electrostatic images on anelectrostatographic imaging surface is well known. The basicelectrostatographic process as taught by C. F. Carlson in US. Pat. No.2,297,691, involves placing a'uniform electrostatic charge on aphotoconductive insulating layer,-exposing the layer to a light andshadow image to dissipate-the charge on the areas of the layer exposedto the lightand developing the resulting latent electrostatic image bydepositing on the image a finely divided electroscopic material referredto in the art as toner. The toner will normally be attracted to thoseareas of the layer which retain a charge, thereby forming a toner imagecorresponding to the latent electroscopic image. This powder image maythen be transferred, usually electrostatically, to a supportsurface suchas paper. The transferred image may subsequently be permanently affixedto the support surface by heat. Other suitable fixing means such assolvent or overcoating treatment may be substituted for the foregoingheat fixing steps.

Many methods are known for applying the electroscopic particles to thelatent electrostatic image to be developed. One development method asdisclosed'by E. N. Wise in US. Pat. No. 2,618,552, is known as cascadedevelopment. In this method, developer material, comprising relativelylarge carrier particles having finely divided toner particleselectrostatically clinging to the surface of the carrier particles, isconveyed to and rolled or cascaded across the latent electrostaticimagebearing surface. The composition of the toner particles is sochosen as to have a triboelectric polarity opposite that of the carrierparticles. In order to de-. velop a negatively charged latentelectrostatic image, I

. develop a positively charged latent electrostatic image,

the electroscopic powder and carrier should be selected in which thepowder is triboelectrically negative in relation to the carrier. Thistriboelectric relationship between the powder and carrier depends ontheir relative positions in a triboelectric series in which thematerials are arranged so that each material is charged with a positiveelectrical charge when contacted with any material below it in theseries and with a negative electrical charge when contacted with anymaterial above it in the series. As the mixture cascades or rolls acrossthe image-bearing surface, the toner particles are electrostaticallydeposited on and secured to the charged portions of the latent image andare not secured to the uncharged or background portions of the image.The cascade development process has the distinct advantage that most ofthe toner particles accidentally deposited on the background portion areremoved by the rolling carrier, due apparently to the greaterelectrostatic attraction between the toner and the carrier than betweenthe toner and the discharged background. The carrier particles andunused toner particles are then recycled. The cascade developmentprocess is extremely good for the development of line copy images, andis the most widely used commercial electrostatographic developmenttechnique. A- general purpose office copying machine incorporating thistechnique is described in US. Pat. No. 3,099,943.

Another technique for developing electrostatic images is the magneticbrush process as disclosed, for example, in US. Pat. No. 2,874,063. Inthis process, a

'developer' material containing tonerand magnetic carrierparticles isattracted to and is carried by a magnet. Y

The magnetic field of the magnet causes alignment of the magneticcarriers in a brush-like configuration.

. This magnetic brush is engaged with an electrostatic image-bearingsurface and the toner particles are drawn from the brush to theelectrostatic image by electrostatic attraction. Many other methods suchas touchdown development as disclosed by C. R. Mayo in US. Rat. No.2,895,847 are known for applying electroscopic particles to the latentelectrostatic image to be developed. The development processes asmentioned above together with numerous variations are well known to theart through various patents and publications and through the widespreadavailability and utilization of electrostatographic imaging equipment.

In automatic electrostatographic reproducing equipment, it isconventional to .employ an electrostatographic imaging surface in thefonn of a cylindrical drum which is continuously rotated through a cycleof sequential operations including charging, exposure, developing,transfer and cleaning. The imaging surface is usually charged withcorona with positive polarity by means of a corona generatingdevice ofthe type disclosed by L. E. Walkup in US. Pat. No. 2,777,957 which isconnected to a suitable source of high potential. After forming a powderimage on the electrostatic image during the development step, the powderimage is electrostatically transferred to a support surface by means ofa corona generating device such as the corona device mentioned above. Inautomatic equipment employing a rotating drum, a support surface towhich a powdered image is to be transferred from the drum is movedthrough the equipment at the same rate as the periphery of the drum andcontacts the drum in the transfer position interposed between the drumsurface and the corona generating device. Transfer is effected by thecorona generating device which imparts an electrostatic charge toattract the powder image from the drum to the support surface. Thepolarity of charge required to effect image transfer is dependent uponthe visual form of the original copy relative to the reproduction andthe electroscopic characteristics of a developing material employed toeffect development. For example, where a positive reproduction is to bemade of a positive original, it is conventional to employ a positivepolarity corona to effect transfer of a negatively charged toner imageto the support surface. When a positive reproduction from a negativeoriginal is desired, it is conventional to employ a positively chargeddeveloping material which is repelled by the charged areas on the plateto the discharge areas thereon to form a positive image which may betransferred by negative polarity corona. In either case, a residualpowder image and occasionally carrier particles remain on the plateafter transfer. Before the plate may be reused for a subsequent cycle,it is necessary that the residual image and carrier particles, ifany,'be removed to prevent ghost images from forming on subsequentcopies. In the positive-to-positive reproduction process descratches onthesurfaces during image transfer and sur- 7 face cleaning operations.

plate surface by a phenomenon that is not fully understood but believedcaused by an electric charge. The

charge is substantially neutralized by means of 'a corona generatingdevice prior to.contact of the residual powder with a cleaning device toenhance the cleaning efficiency of thecleaning device.

Typical electrostatographic cleaning devices include the web-typecleaning apparatus as disclosed, for example, by W. P. Graff, Jr., etal. in U.S. Pat. No. 3,186,838. In the Graff, Jr. et al. patent, removalof the residual powder and carrier particles on the plate is efhighcontrast copies such as letters, tracings and the like, it is desirableto select the electroscopic powder.

and carrier materials so that their mutual electrification is relativelylarge; the degree of such electrification being governed in most casesby the distance between their relative positions in the triboelectricseries. However, when otherwise compatible electroscopic powder andcarrier materials are removed from each other in the triboelectricseries by too great a distance, the resulting images are very faintbecause the attractive forces between the carrier and toner particlescompete Carrier particles must be made'from or coated with materialshaving appropriate triboelectric properties as well as certainother'physical characteristics-Thus, the

I materials employed in the carrier particles should have with theattractive forces between the latent electrostatic image and. the tonerparticles. Although the image density described in the immediatelypreceding sentence may be improved by increasing the toner concentrationin the developer mixture, undesirably high I background toner depositionas well as increased toner impaction and agglomeration is encounteredwhen the toner concentration in the developer mixture is excessive. Ithas been proposed to increase the initial charge on theelectrostatographic imaging surface to improve the density of thedeposited powder image, but this would necessitate an excessively highcharge on the imaging surface to attract the electroscopic powder awayfrom the carrier particle. Excessively high electrostatographic imagingsurface charges are not only undesirable because of the high powerconsumption necessary rier particles thereby leaving many carrierparticles substantially bare of toner particles. Further, adherence ofcarrier particles to reusable electrostatographic imaging surfacepromotes the formation of undesirable a triboelectric value commensuratewith the triboelectric values of the toner and the imaging surface toafford electrostatic transfer of the toner to the carrierparticle andsubsequest transfer of the toner from the carrier particle to the imageon the imaging. surface without excessive power requirements.Furthermore, the triboelectric properties of all the carrierparticlesshould be relatively uniform-to permit uniform pick-up and subsequentdeposition of toner. The materials employed in the carrier particlesalso should have an intermediate hardness so as not to'scratch theimaging surface upon which the electrostatic image is initially placedwhile being sufliciently hard to withstand the forces to which they aresubjected during recycle. The carrier particles as well as the surfacethereof also should not be comprised of materials which are so brittleas to cause either flaking of the surface or particle break-up under theforces exerted on the particles during recycle. The flaking causesundesirable effects in that the relatively small flaked particles willeventually be transferred to the copy surface thereby interfering withthe deposited toner and causing imperfections in the copy image.Furthermore, flaking of the carrier particle surface will cause theresultant carrier particles to have nonunifonn triboelectric propertieswhen the carrier particle is composed of a core material difierent thanthe surface coating thereon. This results in undesirable nonuniformpick-up of toner by the carrier particles and nonunifonn deposition oftoner on the image. In addition, when the carrier particle size isreduced, the removal of the resultant small particles from the imagingsurface becomes increasingly difficult. Thus, the type of materialsuseful for making carrier particles or for coating carrier particles,although having the appropriate triboelectric properties, are limitedbecause other physical properties which they possess may cause theundesirable results discussed above.

It is highly desirable to alter triboelectric properties of the carrierparticles to accommodate the use of desirable toner compositions whileretaining the other desirable physical characteristics of the carrierparticle. The alteration of the triboelectric properties of carrierparticles by applyin'ga surface coating thereon is a particularlydesirable technique. With this technique, not only is it possible toalter the triboelectric properties of carrier particles made frommaterials having desirable physical characteristics, it is also possibleto employ materials previously not suitable as carrier particles. Thus,for example, carrier particles having desirable physical properties withthe exception of hardness, can be coated with a material havingdesirable hardness as well as other physical properties, rendering theresultant product useful as carrier particles. Coating the carrierparticles with the additive to alter the triboelectric propertiesthereof rather than blending the additive into the carrier materialduring initial formation of the carrier particles is preferred sinceless additive need be employed to effect the desired change in thetriboelectric value. Furthermore, the addition of high concentrations ofadditive to the original carrier material to alter the triboelectricvalue thereof requires a major manufacturing operation and oftenundesirably alters the original physical characteristics of the carriermaterial. Thus, there is a present need for providing irnprovedelectrostatographic carrier particles which can have their triboelectricvalue varied over a wide range while retaining desirable physicalcharacteristics of hardness, durability and the like. This is especiallytrue in view of the constant development of new and improved tonercompositions.

It is, therefore, an object of this invention to provide carriermaterials which overcome the above noted deficiencies.

It is another object of the present invention to provide novelelectrostatographic developer compositions.

It is a further object of the presentinvention to provide a method forforming carrier particles of varying triboelectric values whileretaining desirable physical and chemical properties of the originalcarrier particles.

It is a still further object of the present invention to provide carrierparticles and development compositions useful in presently knownelectrostatographic techniques. Further objects of the present inventionwill become evident in view of the following detailed discussion.

The present invention provides carrier particles comprising a corematerial and a surface coating thereon comprising a polymeric celluloseester.

It has been found that polymeric cellulose esters possess suitablephysical properties of durability, hardness, triboelectric value and thelike, to afford their use as coatings for carrier particles useful inelectrostatogra? phy. The use of polymeric cellulose esters isadvantageous in that they can easily be applied as durable coatings tocarrier particles which are resistant to the forces normally encounteredin electrostatographic reproduction. Furthermore, it has been found thatthe triboelectric values of the cellulose esters can be varied over awide range without adversely affecting the properties thereof whichrender them useful in carrier particles. The polymeric cellulose esterswhich are useful in the present invention include cellulose esters whichhave particles. Representative cellulose esters which can be employedinclude: Cellulose acetate, cellulose acetate butyrate, celluloseacetate propionate, cellulose propionate, cellulose triacetate and thelike. The polymeric cellulose esters employed herein have a volumeresistivity of between about and about 10 ohmcentimeters at 23C.

The polymeric cellulose esters can be applied to coat any of thewell-known carrier particle materials. Typical carrier core materialsinclude: Sodium chloride, ammonium chloride, aluminum potassiumchloride, Rochelle salt, sodium nitrate, potassium chlorate, granularzircon, granular silicon, methylmethacrylate, glass, silicon dioxide,flintshot, iron, steel, nickel, carborundum, and mixtures thereof. Manyof the foregoing and other typical carriers are described by L. E.Walkup in U.S. Pat. No. 2,618,511; L..E. Walkup, et al. in U.S. Pat. No.2,638,416 and E. M. Wise in U.S. Pat. No. 2,618,552. An ultimate coatedcarrier particle diameter between about 30 microns to about 1,000microns is preferred for electrostatographic use because the treatedcarrier particle then possesses sufficient density and inertia to avoidadherence to the latent electrostatic images during the development;

In order to apply the polymeric cellulose ester coatings to the carrierparticles, any conventional solvent coating process can be employed.Thus, the polymeric cellulose derivative can be reduced to a liquid orsemiliquid state by dissolving the polymer in a suitable solvent. Y Theuncoated carrier particles are intimately conta cted with the dissolvedpolymeric cellulose ester in order to completely coat the particles. Theconditions of contact, including'temperature, polymer concentration' andcarrier particle concentration, are regulated so that a uniform coatingis applied to the carrier particles thereby forming coated particlesexhibiting a uniform triboelectric value. Similarly, these conditionsare maintained so that the coating thickness does not become excessiveand promote formation of carrier parti-. cle agglomerates. After contactof the particles and the coating material and/or during contact thereof,the mixture can be treated to solidify the coating material I on theparticles as, for example, by evaporating the soletate or celluloseacetate are employed, suitable solvents which can be employed are:ketones, such as acetone, methyl ethyl ketone, isophorone, cyclohexanoneand the like; esters such as methyl acetate, ethyl acetate, ethyllactate, ethylene glycol monomethyl ether acetate and the like;halogenated solvents such as methylene dichloride, ethylene dichlorideand the like; nitroparaffins such as nitroethane, nitromethane and thelike, as well as other solvents such as 1,4-dioxane, diacetone alcoholand the like. Mixtures of the above solvents with alcohols, especiallythe lowerjalkanols containing from one to about six carbon atoms, arealso useful. Such mixtures generally contain from about 10 to about 50percent by volume of the alcohol. Illustrative of such mixtures are:acetonezmethanol (:20), acetonezethanol (:10), methylenedichloride:methanol (80:20), nitroethanezethanol (50:50),nitroethanezethanol (80:20), ethyl acetate:ethanol'(80:20), ethylenedichloridezmethanol 80:20), and the like.

When employing polymeric cellulose esters such as cellulose acetatebutyrate, cellulose acetate propionate and the like, solvents such asketones, for example, acetone, methyl ethyl ketone, cyclohexanone andthe like; esters such as methyl acetate, ethyl acetate, methylCellosolve acetate and the like; halogenated solvents such as methylenechloride, ethylene chloride and the like; nitroparafiins such asnitroethane, l-nitropropane and the like, can be suitably employed.

Helpful guidance in selection of a suitable solvent for a particularcellulose ester can be found in Modern Plastics Encyclopedia, Vol. 46;No. 10A, pages 1006-1007 (October, 1969).

Generally, the polymeric cellulose ester can be dissolved in a suitablesolvent to form a coating solution containing from about to about 20percent by weight solids. Preferably, the solids content is about 10percent by weight.

It is most convenient to remove the solvent from the polymer-carrierparticle mixture by contact with an inert gas stream from which it canbe condensed and recycled for further use.

One distinct advantage of the present invention is that thetriboelectric value of the polymeric cellulose esters employed hereincan be varied over a wide range by incorporating additives into thepolymer composition to be coated on the carrier particle. Thus, in onespecific aspect of the present invention it is contemplated to useadditives having suitable triboelectric values for incorporation in thecoating composition. The additives which can be employed can be discreteparticulate material or they can be dissolved or dispersed in thepolymeric cellulose ester to form homogeneous compositions. When it isdesired to employ cellulose esters having an additive which is to behomogeneously dispersed or dissolved therein, it is preferred to employa coating process utilizing a solvent which is effective for both thepolymeric cellulose ester and the additive.

The polymeric cellulose esters employed as coatings in the presentinvention can contain the conventional additives normally employedtherewith including plasticizers, antioxidants and the like. Suitableplasticizers which can be employed are diesters of adipic acid includingdiethyl adipate, dibutyl adipate, diisobutyl adipate, dicapryl adipate,di-(2-ethylhexyl)adipate; azelaic acid derivatives such asdi-(2-ethylhexyl)azelate, di-n-hexyl azelate and diisooctyl azelate;benzoic acid derivatives such as diethylene glycol dibenzoate anddipropylene glycol dibenzoate; citric acid derivatives such as triethylcitrate, tricyclohexyl citrate and acetyl tri-n-butyl citrate; glycerolderivatives such as glycerol monoacetate, glycerol diacetate, glyceroltriacetate, glycerol tripropionate, glycerol tributyrate, glycerol etheracetate; glycol derivatives such as ethylene glycol dipropionate,ethylene glycol dibutyrate, diethylene glycol dipropionate, triethyleneglycol diacetate, triethylene glycol dipropionate, methyl phthalyl ethylglycolate, ethyl phthalyl ethyl glycolate, butyl phthalyl ethylglycolate; phosphoric acid derivatives such as triethylphosphate,tributylphosphate, and triphenylphosphate; phthalic acid derivativessuch as dimethyl phthalate, diethyl phthalate, dipropyl phthalate,dibutyl phthalate, ditridecyl phthalate, diallyl phthalate; succinicacid derivatives such as diethyl succinate and dibutyl succinate; andtartaric acid derivatives such as diethyl tartarate and dibutyltartarate and the like.

Any suitable pigmented or dyed electroscopic toner material may beemployed with the treated carriers of this invention. Typical tonermaterials include: gum copal, gum sandarac, rosin, cumaroneindene resin,asphaltum, gilsonite, phenol-formaldehyde resins, rosinmodifiedphenol-formaldehyde resins, methacrylic resins, polystyrene resins,polypropylene resins, epoxy resins, polyethylene resins and mixturesthereof. The particular toner material to be employed obviously dependsupon the separation of the toner particles from the treated carrierbeads in the triboelectric series. Among the patents describingelectroscopic toner compositions are U.S. Pat. No. 2,659,670 to Copley;U.S. Pat. No. 2,753,308 to Landrigan; U.S. Pat. No.

, 3,079,342 to Insalaco; U.S. Pat. No. Re 25,136 to Carlson and U.S.Pat. No. 2,788,288 to Rheinfrank et al. These toners generally have anaverage particle diameter between about'l and about 30 microns.

The following examples further define, describe and compare methods ofpreparing the preferred carrier and developer materials of the presentinvention and of utilizing them to develop electrostatic latent images.Parts and percentages are by weight unless otherwise indicated.

In the following examples, the relative triboelectric values generatedby contact of carrier beads with toner particles are measured by meansof a Faraday Cage. The device comprises a brass cylinder having adiameter of 1 inch and a length of 1 inch. A 100-mesh screen ispositioned at each end of the cylinder. The cylinder is weighed, chargedwith 0.5 gram of a mixture of carrier and toner particles and connectedto ground through a capacitor and an electrometer connected in parallel.Dry compressed air is then blown through the brass cylinder to drive allthe toner from the carrier. The charge on the capacitor is then read onthe electrometer. Next, the chamber is reweighed to determine the weightloss. The resulting data is used to calculate the toner concentrationand the charge in microcoulombs per gram of toner. Since triboelectricmeasurements are relative, the measurements should, for comparativepurposes, be conducted under substantially identical conditions. Thus, atoner comprising styrene-n-butyl-methacrylate copolymer, polyvinylbutyral and carbon black by the method disclosed by M. A. Insalaco inU.S. Pat. No. 3,079,342 is used as a contact triboelectrificationstandard in all the examples. Obviously, other suitable toners such asthose listed above may be substituted for the toner used in theexamples.

EXAMPLE 1 Coated glass carrier particles were made and tested asfollows: 800 grams of Potters No. 6 glass beads were placed in atumbling barrel type mixer. 72 grams of a 10 percent cellulose acetatebutyrate solution (EA- B-l71-2 manufactured by Eastman Kodak Co,Rochester, N.Y., approximate butyryl content 17 percent) in ethylenedichloride solvent was charged to the tumbling barrel mixer. Theresultant mixture was mixed for about 1 hour at a temperature of about50C. During mixing, hot air was directed into the barrel at atemperature of about 80C. for 15 minutes to evaporate the solvent.Thereafter, cold air at a temperature of about 25C. was directed intothe barrel to solidify the cellulose acetate butyrate coating on thebeads. The coated beads dried very well with only a small percentage ofagglomerates. The beads were then screened through a No. 25 mesh screenand onto a 35 mesh screen to obtain glass carrier beads uniformly coatedwith cellulose acetate butyrate.

The triboelectric value of the carrier beads was determined inaccordance with the method described above and were found to have avalue of about 13.8 micro-coulombs per gram of toner. The coated beadswere mixed with a toner comprising styrene-n-butyl methacrylatecopolymer, polyvinyl butyral and carbon black to form a developingcomposition in a weight ratio of parts carrier beads to 1 part toner forevaluation. The test was conducted in a Model D electrostatographicapparatus (manufactured by Xerox Corporation, Rochester, NY.) and thequalities of the cop- EXAMPLE 2 Coated flintshot carrier particles aremade and tested as follows: pounds of 2035 mesh flintshot was placed ina tumbling barrel type mixer. 204 grams of a per-.

, mixing, hot air was directed into the barrel at a temperature of about70C. for about 10 minutes to evaporate the solvent. Thereafter, cold airat a temperature of about 24C. was directed into the barrel to solidifythe cellulose acetate butyrate coating on the beads. The coated beadsdried very well with only a small percentage of agglomerates. The beadswere then screened through a 25 mesh screen and onto a 35 mesh screen toobtain flintshot carrier beads uniformly coated with cellulose acetatebutyrate.

The triboelectric value of the carrier beads was determined inaccordance with the method described above and were found to have avalue of about 10.4 microcoulombs per gram of toner. The coated beadswere mixed with the toner composition described in Example l to form adeveloping composition in a weight ratio of 100 parts carrier beads to 1part toner for evaluation. The test was conducted in a Model D"electrostatographic apparatus and the qualities of the copies producedtherefrom were evaluated. In every respect, including toner pick-up,resolution and overall quality, the copies so obtained were rated good.

EXAMPLE 3 Coated flintshot carrier particles were made and tested asfollows: 5 pounds of 2035 mesh flintshot were placed in a tumblingbarrel type mixer. 204 grams of a 10 percent solution of celluloseacetate butyrate (EAB-171-2) containing 2 grams of Hansa Yellow inethylene dichloride solvent were charged to the tumbling barrel mixer.The resultant mixture was mixed for about 1.5 hours at a temperature ofabout 50C. During mixing, hot air was directed into the barrel at atemperature of about 60C. for minutes to evaporate the solvent.Thereafter, cold air at a temperature of about 25C. was directed intothe barrel to solidify the cellulose acetate butyrate coating on thebeads. The coated beads dried very well with only a small percentage ofagglomerates. The beads were then screened through a 25 mesh screen andonto a 35 mesh screen to obtain carrier beads uniformly coated withcellulose acetate butyrate.

The triboelectric value of the carrier beads was determined inaccordance with the method described above and found to have a value ofabout 11.6 microcoulombs per gram of toner. The coated beads were mixedwith the toner described in Example I to form a developing compositionin a weight ratio of 100 parts carrier beads to 1 part toner forevaluation. The test was conducted in a Model D electrostatographicapparatus and the qualities of the copies produced therefrom wereevaluated. In every respect, includtoner pick-up, background resolutionand overall quality, the

copies so obtained were rated good.

EXAMPLE 4 Coated flintshot carrier particles were made and 7 tested asfollows: 5 pounds of 35 mesh flintshot were placed in a tumbling barrelmixer. 204 grams of a 10 percent cellulose acetate butyrate solution(EAB 500-1 manufactured by Eastman Kodak Company,

- Rochester, N.Y., approximate butyryl content 50 percent) in ethylenedichloride solvent as well as 2 grams Luxol blue dye were charged to thetumbling barrel mixer. The resultant mixture was mixed for about 1.5hours at a temperatureof about 55C. During mixing, hot air was directedinto the barrel at a temperature of about C. for about 15 minutes toevaporate the solvent. Thereafter, cold air at a temperature of about22C. was directed into the barrel to solidify the cellulose acetatebutyrate coating beads. The coated beads dry very well with only a smallpercentage of agglomerates. The beads were then screened through a 25mesh screen and onto a 35 mesh screen to obtain flintshot carrier beadsuniformly coated with cellulose acetate butyrate. I

The triboelectric value of the carrier beads was determined inaccordance with the method described above and were found to have avalue of 3.9 microcoulombs per gram of toner. The coated beads'weremixed with the toner composition described in Example 1 to form adeveloping composition in a weight ratio of parts carrier beads to 1part toner for evaluation. The test was conducted in a Model Delectrostatographic apparatus and the qualities of the copies producedtherefrom were evaluated. In every respect, including toner pick-up,resolution and overall quality, the copies so obtained were rated fairwith a slight background.

Although specific materials and conditions were set forth in the aboveexemplary processes in making and using the carrier materials of thisinvention, these are merely intended as illustrations of the presentinvention. Various other toners, carrier cores, substituents andprocesses such as those listed above may be substituted in the exampleswith similar results. 7

Other modifications of the present invention will occur to those skilledin the art upon a reading of the present disclosure. These are intendedto be included withinthe scope of this invention.

What is claimed is:

1. An electrostatographic developer mixture comprising finely dividedtoner particles electrostatically clinging to the surface of carrierbeads, each of said carrier beads comprising a core having a unifonncoating thereon comprising a polymeric cellulose ester wherein saidcoating exhibits a volume resistivity of between about l0 and about 10ohm-centimeters at 23C.

2. An electrostatographic developer mixture according to claim I whereinsaid polymeric cellulose ester is cellulose acetate butyrate.

3. An electrostatographic developer mixture according to claim 1 whereinthe carrier bead core is glass.

4. An electrostatographic developer mixture according to claim 1 whereinthe carrier bead core is flintshot.

5. An electrostatographic imaging process comprising the steps offorming an electrostatic latent image on a surface and developing saidelectrostatic latent image by contacting said electrostatic latent imagewith an Ill electrostatographic developer mixture comprising finelydivided toner particles electrostatically clinging to the surface ofcarrier beads, each of said carrier beads comprising a core having auniform coating thereon comprising a polymeric cellulose ester whereinsaid coating exhibits-a volume resistivity of between about and about 10ohm-centimeters at 23C., whereby at least a portion of said finelydivided toner particles are attracted to and held on said surface inconformance with said electrostatic latent image.

6. An electrostatographic imaging process comprising the steps'offorming an electrostatic latent image on a surface and developing saidelectrostatic latent image conformance with said electrostatic latentimage.

1. AN ELECTROSTATOGRAPHIC DEVELOPER MIXTURE COMPRISING FINELY DIVIDEDTONER PARTICLES ELECTROSTATICALLY CLINGING TO THE SURFACE OF CARRIERBEADS, EACH OF SAID CARRIER BEADS COMPRISING A CORE HAVING A UNIFORMCOATING THEREON COMPRISING A POLYMERIC CELLULOSE ESTER WHEREIN SAIDCOATING EXHIBITS A VOLUME RESISTIVITY OF BETWEEN ABOUT 10**10 AND ABOUT10**14 OHMCENTIMETERS AT 23*C.
 2. An electrostatographic developermixture according to claim 1 wherein said polymeric cellulose ester iscellulose acetate butyrate.
 3. An electrostatographic developer mixtureaccording to claim 1 wherein the carrier bead core is glass.
 4. Anelectrostatographic developer mixture according to claim 1 wherein thecarrier bead core is flintshot.
 5. An electrostatographic imagingprocess comprising the steps of forming an electrostatic latent image ona surface and developing said electrostatic latent image by contactingsaid electrostatic latent image with an electrostatographic developermixture comprising finely divided toner particles electrostaticallyclinging to the surface of carrier beads, each of said carrier beadscomprising a core having a uniform coating thereon comprising apolymeric cellulose ester wherein said coating exhibits a volumeresistivity of between about 1010 and about 1014 ohm-centimeters at23*C., whereby at least a portion of said finely divided toner particlesare attracted to and held on said surface in conformance with saidelectrostatic latent image.
 6. An electrostatographic imaging processcomprising the steps of forming an electrostatic latent image on asurface and developing said electrostatic latent image by contactingsaid electrostatic latent image with an electrostatographic developermixture comprising finely divided toner particles electrostaticallyclinging to the surface of carrier beads, each of said carrier beadscomprising a core having a uniform coating thereon comprising celluloseacetate butyrate wherein said coating exhibits a volume resistivity ofbetween about 1010 and about 1014 ohm-centimeters at 23*C., whereby atleast a portion of said finely divided toner particles are attracted toand held on said surface in conformance with said electrostatic latentimage.